An 11-year-old female was referred to our hospital with a giant mediastinal mass occupying the entire left hemithorax. Percutaneous biopsy showed no evidence of immature components or malignant cells. Since the tumor compressed the vital structures of the mediastinum with total atelectasis of the left lung, we performed median sternotomy with left anterior thoracotomy. The tumor was punctured, and part of its fluid content was aspirated to achieve reduction in the size of the mass, making tumor resection easier. The tumor was totally resected without complications. A mediastinal tumor occupying the entire hemithorax is uncommon; its surgical strategy is discussed in this manuscript.